A floating flap gate exists which is disposed at an opening of a seawall and blocks the opening at the time of a rising water in order to prevent the rising water from flowing into living spaces or underground spaces, by causing a door body to float, using a buoyancy of the water which is trying to flow in (e.g., Patent Reference 1).
However, the floating flap gate disclosed in Patent Reference 1 has a problem in that if a speed of the inflowing water is rapid, the floating action of a door body 1 is delayed, resulting in an overflow of water into living spaces or underground spaces (see FIG. 10(a)).
In addition, when the water level drops, the door body 1 stays at a raised state up to a water level which is about ⅓ the height of the door body 1, and subsequently exhibits a hazardous behavior such as suddenly falling (see FIG. 10(b)).
In order to prevent the problem of overflow during the initial influx of water, there was proposed a floating flap gate with a rope having a counterweight attached to one end, and with the other end connected to the door body via a pulley (e.g., Patent Reference 2).
The floating flap gate disclosed in Patent Reference 2 solves the problem of the delayed floating action of the door body during the initial influx of water by compensating for an insufficient buoyancy of the floating flap gate by using the weight of a counterweight.
However, the floating flap gate disclosed in Patent Reference 2 does not readily lower when the water level drops, because weight of the counterweight continually operates in a direction which assists in the floating action of the door body.
Moreover, in order to avoid a sudden falling action, which was one of the problems described above, a system was disclosed in Patent Reference 3, which employs a damper circuit to dampen the falling speed while the door body is being lowered. However, in the case of the system disclosed in Patent Reference 3, there is a risk of generating an overflow at the time of an initial influx, because the damper circuit dampens the rising speed while the door body is rising.    Patent Reference 1: Japanese Patent Application Kokai Publication No. 2001-214425    Patent Reference 2: Japanese Patent Application Kokai Publication No. 2003-253912    Patent Reference 3: Japanese Patent No. 4,388,494